The high prevalence of physical inactivity among middle- and older-aged adults is related to a number of chronic diseases and decreased quality of life. Counselor-directed physical activity programs may enhance extrinsic motivation (social influence theory) and promote physical activity. However, social influence may undermine long- term intrinsic motivation (cognitive evaluation theory), contributing to physical activity relapse once counselor-initiated contact ends. In contrast, programs that diminish the socially evaluative and controlling aspects of the counseling interchange may promote intrinsic motivation and foster long-term maintenance of physical activity, even when counselor-initiated contact ceases. The proposed study compares these alternative theoretical perspectives by conducting a randomized controlled clinical trial examining a 12-month telephone-administered counseling program delivered either by a person (social influence enhancement) or computer (cognitive evaluation enhancement) on one-year participation in and 18-month maintenance of physical activity in older sedentary adults. Outcome measures include physical activity and performance measures, as well as quality of life. Possible moderators and mediators of the response to intervention will be explored, as will costs associated with each intervention program.